It has been known for many years that the inclusion of small quantities of paraffin wax in polyester resins prevents are inhibition of the open surface of mouldings on cure (Brit. Pat. No. 713,332). The wax works by migration to the open surface forming a thin layer which prevents air from interfering with surface cure. The quantity needed depends on the melting point of the wax used, commonly a paraffin, although other waxy materials have been claimed, e.g. stearyl stearate (Brit. Pat. No. 850,762). In practice about 100 ppm. of a paraffin wax mp 52.degree.-54.degree. C. has been employed.
More recently a need has arisen on environmental grounds to reduce the evaporation of styrene monomer from the surface of a laminate in the interval between lay-up and cure. This can be achieved by the incorporation of higher levels of paraffin wax, and the effect of different levels of wax on the styrene loss is progressively greater as the levels of paraffin wax increase up to about 800 ppm. for most types of resin, although some types will require even more wax.
However, at the higher levels of wax required to substantially reduce styrene evaporation several drawbacks have been noted:
(i) the wax can separate from the liquid resin when it is stored at low temperatures, PA1 (ii) the layer of wax can interfere with the adhesion between one layer of laminate and the next, especially where there is a delay between lay-up and cure.
Several methods of overcoming these disadvantages have been proposed i.e. the incorporation of small quantities of a straight chain hydrocarbon, e.g. n-octane, or the use of surfactant such as a polyoxyethylene sorbitan fatty acid ester (German Offenlegungsschrift No. 2554930).
Neither of these methods has been particularly successful. For example 2% n-octane lowers the heat deflection temperature of the cured resin and imparts a milky appearance to the laminate.
It has been proposed (GB No. 2011918A) to use, as adhesion promoting agent, an acyclic, hydrophilic ether or ester, said ether or ester having at least two terminal hydrocarbon groups with at least one double bond in each hydrocarbon group, for example, triethyleneglycol dimethacrylate, an unsaturated, isoprenoidal-compound, for example, dipentene, or an ether or ester of said isoprenoidal compound. It is to be noted that none of the compounds disclosed in GB 2011918A contains a free carboxylic acid group; all of the carboxyl groups of such compounds are esterified. The adhesion promoting agent disclosed in that specification is applied there only to one polyester resin composition.
In our copending European Patent Application No. 79.301587.6 we have described a wide range of unsaturated polyester resin compositions for ambient temperature open lay-up moulding containing styrene as an agent for cross-linking during curing, a wax for preventing air inhibition and, as adhesion promoter, a compound of the formula: ##STR2## where R is a straight or branched chain aliphatic C.sub.6 -C.sub.24 hydrocarbon, ##STR3##
These resin compositions are designed to allow efficient reduction of styrene evaporation enabling the addition of the larger quantities of paraffin wax or other ingredient which are needed in comparison to the quantities which would have been needed merely for the prevention of air inhibition (air inhibition is an effect of the atmosphere on the surface layer of the laminate so that it does not cure properly), without however, involving the penalty of loss of adhesion or spoiled appearance. They achieve this by promoting inter-laminar adhesion to the extent that they overcome the disadvantages which would otherwise have arisen from the presence of such larger quantities of waxy substance. Furthermore, the presence of such compounds will permit the preservation of the desired heat deflection temperature in the cured resin. The compound is mixed with the conventional components of an unsaturated polyester resin composition (which conventionally consists of a prepolymer plus styrene as a vehicle and cross-linking agent and an accelerator, to which is then added a catalyst when curing is desired), and is theoretically capable of taking part in the cross-linking reaction. The adhesion promoters are applicable to a wide range of polyester resin compositions including those based on isophthalic acid.